I have been in the past have to oportunity to use several lever espresso machines such as La Pavoni profesional, La Pavoni euripicola which is essentially a smaller capacity version of the profesional and a Ponte Vecchio Lusso. These two three espresso machines essentially utilize single boilers and through cleaver of designs for both brewing espresso and steaming milk. I also have restore and still own a monster of an espresso machine, a La Marzocco Linea three groups. It is truly a gorilla, weighting two-hundred fifty something pounds and requireing 7000 Watts of power for its operation; I consider myself clinically insanse for using this thing at my home!!!
The La Marzocco in constrast to the levers is a double boilers espresso machine with 2 dedicate boilers, one to brewing coffee and one to steaming milk. It is consider the most ideal design by most people in the coffee industry. However, I never had a chance to use another type of espresso machine before and that is a Heat-Exchanger espresso machine. In princliple, a heat-exchanger (or HX for short) combine the idea of single and double boiler into one, a relatively smaller boiler within a big boiler! The big boiler is for steaming and also for providing heat to the small boiler, the small boiler is for heating water to coffee brewing temperature. The small boiler in this case is a Heat-exchanger!
In the next few week or so, I'm looking forward to doing comparison of my new coffee toy, the NS Musica which is an HX espresso machine to my other espresso machine. It is going to be exciting!!!